Generally, thermal recording materials having a thermal recording layer based mainly on a colorless or pale-colored basic (electron-donating) leuco dye (hereinafter sometimes to be simply abbreviated as “dye” or “leuco dye”) and an electron-accepting developer that develops color by reacting with dye on heating (hereinafter sometimes to be simply abbreviated as “developer”) are widely used in practice. For recording on the thermal recording materials, a thermal printer with a built-in thermal head, and the like are used. This method of thermal recording is advantageous over other conventional recording methods in practical use, with features such as (1) noiselessness during recording, (2) obviation of the need for development and fixation, (3) freedom from maintenance work, (4) relatively inexpensive instrumentation, (5) compactness, and (6) very vivid colors developing in the images obtained, and is widely used for facsimiles, computer terminal printers, automated ticket machines, measurement recorders, handy terminals for outdoor use, and the like. When color-developing sensitivity of such thermal recording materials is insufficient, the color-developing sensitivity needs to be increased, since power consumption increases and printing speed decreases.
A factor having a great influence on the color-developing sensitivity is selection of the dye and developer constituting the thermal recording layer, and an influence of the selection of the developer is particularly high.
For example, benzyl p-hydroxybenzoate expected at one time as a high sensitive developer is no longer used at present, since sensitivity is high but image storability is markedly low. In addition, bisphenol A (4,4′-isopropylidenediphenol) is scarcely used in Japan, since color-developing sensitivity is insufficient, image storability such as plasticizer resistance and heat resistance is not satisfactory, and further, environment hormone activity is suspected. To improve these developers, bisphenol S (4,4′-dihydroxydiphenylsulfone) has been put into practice, but it has defects of high melting point and low color-developing sensitivity at low energy application (i.e., start-up sensitivity).
As a developer expected to achieve high sensitivity, a phenolsulfonic acid ester compound disclosed in patent document 1 is substituted at p-position and shows high sensitivity, but is defective in that the storability of blank area and images in the recorded area is low.
The sulfonate compound disclosed in patent document 2 is not a developer but a sensitizer. The compound specifically disclosed in patent document 2 does not have an electron-accepting group and does not show a developing action.
While patent document 3 discloses bis(phenolsulfonic acid) ester compounds, all of the compounds show high storability of images in the recorded area, but the melting point is high and the color-developing sensitivity at low energy application (that is, start-up sensitivity) is low.